Pressure calculated by using a vacuum as the zero point and including the gauge
and atmospheric pressure in the calculation.
bottom time (ABT):
Total elapsed time in minutes from leaving the surface until ascent is initiated.
A hormone secreted by the adrenal gland into the circulatory system which stimulates
the heart, blood vessels and respiratory system.
(Automatic Deflation Valve): Device on a buoyancy compensator that allows for
rapid air purging.
A gas mixture containing 21% oxygen, 78% nitrogen, and 1% other gasses (mainly
argon); compressed air is used for recreational scuba diving.
A machine that compresses or pressurizes air; for scuba purposes, air is compressed
from the atmospheric level (14.7 psi at sea level) to the capacity of the tank,
usually between 2500-3000 psi.
A condition that occurs when air enters the bloodstream through ruptured alveoli
into the pulmonary capillaries. The air in the bloodstream then forms bubbles,
which can block blood flow to the body's tissues.
The force per unit area exerted by the weight of air; at sea level the air pressure
is 14.7 psi. (air pressure decreases with altitude.)
A set of equations incorporated into diving computers in order to compute nitrogen
uptake and elimination from changes in depth and elapsed time.
an International maritime signal flag, meaning, 'Diver down, keep clear'.
A device a diver can use in place of the primary regulator, in order to make
an ascent while still breathing normally.
Un-even release of pressure from the inner ear. Causing vertigo, dizziness and
An illness brought on by the sudden reduction in pressure of ascent to altitude.
It is the available sunlight underwater used as a source of illumination.
The surrounding pressure; on land, comes from the weight of the atmosphere (see
air pressure), at depth, comes from the weight of the water plus the weight
of the atmosphere. One atmosphere is about 14.7 pounds of pressure per square
Device that uses a needle moving around a dial to provide information.
American Nitrox Divers Incorporated
Medications that reduce the clotting ability of the blood. Particularly dangerous
to divers due to barotrauma of air-filled body cavities.
(Advanced Open Water).
Any object wholly or partly immersed in fluid will be buoyed up by a force equal
to the weight of the fluid displaced by the object.
An inert gas that makes up less than one percent of air (sometimes used as a
dry suit gas).
Irregularities in the rhythm and rate of the heart, particularly dangerous to
divers due to the underwater environment.
The condition characterized by bubble(s) of air from a ruptured lung segment
under pressure; the bubbles enter the pulmonary circulation and travel to the
arterial circulation, where they may cause a stroke.
any means by which an alternating increase and decrease in chest volume is artificially
created while maintaining an open airway in mouth and nose passages; mouth to
mouth, mouth to nose and mouth to snorkel resuscitation are examples.
An extra cylinder of air used on deep dives to allow decompression stops without
fear of running out of air.
Line suspended from a boat or a buoy for a diver to use to control their rate
of ascent or descent.
A common condition manifested by narrowing of air passages within the lungs.
One reason for the narrowing is excess mucous in the airway.
Atmosphere absolute; 1 ata is the atmospheric pressure at sea level; is measured
with a barometer.
The blanket of air surrounding the earth, from sea level to outer space. Also,
a unit of pressure; "one atmospheres is pressure of the atmosphere at sea
level, I.e., 760 mm Hg. Two atmosphere is twice this pressure, 1520 mm Hg, etc.
The ambient pressure including the air column over the water. The air column
= 1 atm. at sea level. In sea water, another atmosphere is added each 33 FSW
(Feet of Sea Water) . There is an increase in pressure per foot of sea water
equivalent to 1/33 or .03030303 . So ATA may be calculated by multiplying the
depth (FSW) by .0303030 and then adding 1 for the air above the water. i.e.
the ATA at 46 FSW = (46 * .0303030) + 1 = 2.3939 ATA. to convert ATA to FSW.
ATA - 1 * 33 = FSW.
Pressure of the atmosphere at a given altitude or location.
Australian Underwater Federation
Aquatic World Awareness, Responsibility and Education. A PADI nonprofit environmental
foundation that provides financial support for aquatic preservation endeavors,
develops conservation-oriented educational materials and initiates public awareness
An axial scrubber is a scrubber design in which the breathing gases move from
top to bottom (or vise-versa) through the scrubber.
When suspended particles in the water, such as sand, are illuminated
by light from a flash, they reflect the light back to the lens. The particles
appear as specks or snow in the photo.
means of entering the water in SCUBA gear from a sitting position such as from
the gunnels of a boat whereby the diver, while securely holding his mask, leans
backward and rolls into the water onto his tank and shoulders. Checking for
an all clear is recommended.
training technique used in some SCUBA classes wherein the student jumps into
the pool while holding all equipment in hand and then dons the equipment on
the bottom of the pool; or, pertaining to or consisting of a means for relieving
an emergency situation.
Same as atmospheric pressure with the exception that it varies with the weather.
Any disease or injury due to unequal pressures between a space inside the body
and the ambient pressure, or between two spaces within the body; examples include
arterial gas embolism and pneumothorax.
or BCD: buoyancy control device
See also buoyancy compensator.
A form of decompression sickness caused by dissolved nitrogen leaving the tissues
too quickly on ascent; is manifestation of decompression sickness.
A pouch within a Buoyancy Compensator which holds the amount of air the diver
desires to provide proper buoyancy.
Garment that provides full length abrasion protection.
A piece of foot protection, usually made of neoprene, worn inside an open-heeled
fin; serves to protect the diver's feet while walking to and from the dive site
and prevents blisters from the fins while swimming; also provides warmth, depending
on thickness. May come in a varying sole thickness.
The time between descending below the surface to the beginning of ascent.
The front end of a boat.
At a fixed temperature for a fixed mass of gas, pressure times volume is a constant
Diving without life support apparatus, while holding one's breath.
British Thermal Units or calories; measurement of heat.
A collection of air or gas surrounded by a permeable membrane through which
gases can enter or exit.
Sharing of the same air supply by two or more divers; an emergency technique
used when one person's air supply is exhausted or unavailable due to equipment
The upward force exerted on an object in liquid, whether the object sinks or
floats. Objects that float are positively buoyant, those that sink are negatively
buoyant and those that stay where placed are neutrally buoyant.
An inflatable vest worn by the diver that can be automatically or orally inflated
to help control buoyancy; abbreviated BC or BCD (Buoyancy Control Device).
Thin copper disk held in place with a vented plug. Designed to rupture if tank
pressure is greatly exceeded.
Made up of a small tube. Uses Boyle's law to determine depth.
CO2; an odorless, tasteless gas that is a byproduct of metabolism; is excreted
by the lungs in exhaled air.
Problems resulting from buildup of CO2 in the blood; they may range from headache
and shortness of breath, all the way to sudden blackout.
CO; odorless, tasteless, highly poisonous gas given off by incomplete combustion
of hydrocarbon fuels.
CO bonds with hemoglobin and prevents blood cells from carrying oxygen. This
causes oxygen deprivation in the tissues and can even cause death.
Illness from inhaling excess CO; problems may range from headache to unconsciousness
Requiring much specialized training and equipment, this involves diving into
an overhead environment including caves, abandoned mines or quarries and natural
springs or sinkholes where the exit is not always visible. "Overhead environment"
means a structure exists which prevents the diver from making a direct vertical
ascent to the surface.
Requiring specialized training, this involves diving into an overhead environment
including caves, abandoned mines or quarries and natural springs or sinkholes;
differs from Cave Diving in that the exit should always be visible. "Overhead
environment" means a structure exists which prevents the diver from making
a direct vertical ascent to the surface.
Refers to a divers certification card for a specific level of achievement.
Course Director. Level of instructor certification authorized to conduct instructor
Centers for Disease Control and Prevention. An U.S. government agency within
the Department of Health and Human Services which, among other functions, maintains
the Traveler Hotline with information on geographic distribution of diseases
and inoculations required/recommended for travel toother countries.
Metric unit for temperature. C=(F-32) x .556
cubic foot. A measure of volume. Scuba cylinders are manufactured in standard
sizes, such as 30, 50, 72 and 80 cf.
The amount of change in either volume or pressure of a given volume of gas is
directly proportional to the change in the absolute temperature.
Apparatus designed to allow divers to re-breath exhaled air after removal of
CO2 and addition of supplemental 02. In contrast to "Open Circuit",
closed circuit scuba is noiseless and produces no bubbles.
A theoretical division of the body with an arbitrarily assigned half time for
nitrogen uptake and elimination. In designing decompression tables the body
is divided into finite number of compartments for purposes of making calculations.
A device that monitors nitrogen in the body during a dive though mathematical
algorithms. The device allows divers to multilevel dive and extend bottom time
beyond what a dive table allows.
Invertebrates that secrete an internal, hard skeleton structure composed of
calcium carbonate, which is absorbed from the surrounding water.
The internal temperature of the body, 98.6F is the normal temperature of the
human body. Deviation from this temperature even a few degrees could be life
An overlapping waistband with Velcro used to secure a Buoyancy Compensator snugly
around the diver's waist.
A horizontal movement of water; currents can be classified as tidal and nontidal;
tidal currents are caused by forces of the sun and moon and are manifested in
the general rise and fall occurring at regular intervals and accompanied by
movement in bodies of water; nontidal currents include the permanent currents
in the general circulatory systems of the sea as well as temporary currents
arising from weather conditions.
Divers Alert Network. Nonprofit organization that provides emergency and informational
advice and assistance for diving injuries, promotes diving-related medical research
and education, collects injury statistics, and offers dive safety services to
its members and the diving community.
The total pressure exerted by a mixture of gases is equal to the sum of the
pressures of each gas of the different gases making up the mixture. Each gas
acting as if it were alone were present and occupied the total volume.
Any change from one ambient pressure to a lower ambient pressure, always results
in a reduction of gas pressure within the body.
Any dive where the diver is exposed to a higher pressure than when the dive
began, the decompression occurs as the diver ascends.
DCI; a term to encompass all bubble-related problems arising from decompression,
including both decompression sickness and arterial gas embolism.
DCS; a general term for all problems resulting from nitrogen leaving the body
when ambient pressure is lowered. Can be divided into Type I (musculoskeletal
and/or skin manifestations only) or the more serous Type II (neurologic, cardiac,
and/or pulmonary manifestations).
On ascent from a dive, a specified time spent at a specific depth, for the purpose
of nitrogen off-gassing. When not mandatory it is called a safety stop.
For recreational divers a deep dive is a dive below 60 ft.
Dive Equipment & Marketing Association. Not-for-profit organization of equipment
manufacturers, training agencies, dive media, travel companies and dive retailers
that seeks to promote scuba diving and snorkeling to the general public.
A device that indicates how far a diver is below the surface.
A line suspended from a boat, float or buoy used to permit divers to control
their descents and ascents and to provide guidance to the bottom in poor visibility
or strong currents; particularly useful on ascent to assist divers to make safety
or emergency decompression stops between 10 and 15 feet.
A dividing membrane or thin partition; the thin muscle separating the chest
cavity from the abdominal cavity; the rubber (or other material) separating
the demand chamber in a regulator from the surrounding water.
Deutsches Institut fur Normung. Design of tank valve popular in Europe in which
the first-stage regulator screws into the tank valve. Recommended for high pressure
Device that constantly measures depth and time, based on a pre-programmed algorithm,
the computer calculates tissue nitrogen uptake and elimination in several theoretical
compartments and provides a continuous readout of the dive profile, including:
depth, elapsed time of the dive, duration at current depth before decompression
becomes mandatory, and a warning if the rate of ascent is too fast.
May be either a red rectangle with a white diagonal stripe or a blue and white
double tailed pennant. Flags are used to warn watercraft to stay away because
there are divers below.
Specially designed underwater lights used for night, cave or wreck diving.
A printed collection of dive times for specific depths, by which the divers
can avoid contacting DCS. Most tables are based on Haldanian theory for nitrogen
up-take and elimination.
Motorized vehicle used by divers to cove long distances underwater without having
Department of Transportation. U.S. government agency that regulates the manufacture,
testing and transport of compressed gas containers, including scuba cylinders.
DOT stamp appears on scuba tanks, followed by the alphabetic designation for
the steel or aluminum alloy the tank is made of and the maximum fill pressure.
Diver propulsion vehicle, underwater scooter that allows a dive to cover an
increased distance underwater. Popular at some resorts.
A water-tight garment that keeps the diver's body warm by providing insulation
with a layer of gas, such as air, for diving in waters that are too cold for
comfortable wetsuit protection, usually below 65'F.
Enriched Air Nitrox. A N2/O2 (nitrogen/oxygen) breathing gas containing more
oxygen (typically 32 or 36 percent) and less nitrogen than plain air. Used by
recreational divers to increase either bottom time or safety margin by decreasing
the amount of nitrogen absorbed. Requires predive testing of gas mixture and
adherence to strict depth restrictions.
A movement of tidal current away from shore or down a tidal stream; tide that
is flowing out or causing a lower water level.
A circular movement of water, in a comparatively limited area, formed on the
side of a main current; may be created at a point where the mainstream passes
a projection or meets an opposite current.
The act of forcing air into an open space to offset increasing water pressure.
A short tube connecting the back of the nose to the middle ear. If clogged,
by mucus, equalization is next to impossible.
Garment worn to prevent decreases in core body temperature and abrasions. Protection
can range from thin body suits to heavy dry suits.
absorption and elimination:
Dissolved gases such as nitrogen are absorbed into the blood and tissues during
the course of the dive. The level of saturation depends on the depth of the
dive. The elimination of these gases is very important in preventing decompression
sickness. The length of time required for elimination depends on the duration
and depth of the dive.
Laws that predict how gases will behave with changes in pressure, temperature
Pressure exclusive of atmospheric pressure, when diving, gauge pressure is due
to the water pressure.
The most common method of entering water from a boat transom, pier, etc., where
the standing diver takes a large step into the water while securely holding
mask, tucking chin and bringing fins quickly together to keep himself at the
surface for a controlled descent.
global positioning system. A worldwide system of navigation based on a ring
of stationary satellites. Small, even handheld, GPS devices can be used to accurately
determine speed and direction of travel, and pinpoint dive site locations.
Related to Haldane's theory that nitrogen is absorbed up and released in an
exponential manner during a dive, and that there is some safe ratio of pressure
change for ascent.
Half the time it takes for a dissolved gas in a tissue (such as nitrogen) to
equililbrate to a new pressure, or to reach full saturation at a new pressure.
Theoretical tissue half times are used in designing dive tables and algorithms
for dive computers.
Restroom on a boat.
Mixture of helium and oxygen, usually reserved for very deep diving.
Second lightest gas; does not cause problems of narcosis to the same extent
as seen with nitrogen, and is therefore used for very deep diving.
The amount of any given gas that will dissolve in a liquid at a given temperature
is a function of the partial pressure of the gas in contact with the liquid
and the solubility coeffient of the gas in the liquid.
Is done in mountain lakes or other high altitude waters at or greater than 1,000
feet (300 meters) above sea level with increased risk of decompression sickness
because of lower-than-sea-level atmospheric pressure at the surface; regular
dive tables and some dive computers and depth gauges are inaccurate above sea
level; special high altitude dive tables and recalibration of gauges and computer
are required; specialty courses are available due to the complexity and added
hazards of this activity.
pressure nervous syndrome:
Abbr. HPNS; A condition which results from breathing Helium under high pressures.
Early symptoms of HPNS are somtimes seen as shallow as 300FSW but more commonly
over 600FSW. The severity also depends on the mix of breathing gases, Nitrogen
can often moderate the affects of HPNS. The early symptoms include muscle tremors,
followed by changes in electroencephalogram (EEG) readings, impaired motor and
problem solving skills. Other symptoms can include euphoria, nausea, vomiting,
lack of appetite and drowsiness. Symptoms sometimes moderate or entirely dissapear
with continued exposure.
The Hogarthian configuration is named after Bill 'Hogarth' Main. It is based
on reducing equipment to a minimum streamlined configuration that nevertheless
includes sufficient redundancy for extended decompression dives.
Garment worn over the head to reduce thermal loss.
A surface-supplied compressed air apparatus, for use in shallow diving in calm
waters. The air is delivered to one or more divers through a long hose.
The common term for the hydrostatic test required on scuba cylinders every five
years to determine whether the tank walls are still strong enough for safe usage.
An inert gas, and lightest of all the elements, has been used in experimental
Pressure test in which the tank is filled with water instead of air and raised
to five thirds the maximum working pressure, causing the water to expand and
Air-tight chamber that can simulate the ambient pressure at altitude or at depth,
is used for treating decompression illness.
A higher than normal P02level in the blood.
In general, these terms relate to a more than a normal amount of Oxygen. Hyperoxic
refers to a mixture of gases with higher than normal Oxygen content (above 21%).
Hyperoxia is the physiological condition associated with breathing too high
of a partial pressure of Oxygen. The human body has a limit on both the partial
pressure of Oxygen it can tolerate and the long term dosage of Oxygen. The partial
pressure upper limit is generally considered to be approximately 1.6 ppO2 but
most divers leave some margin for error and a more typical upper limit is 1.4
ppO2. When high partial pressures of Oxygen are inspired, convulsions may occur
with little or no warning.
A body temperature warmer than normal, less common in diving than Hypothermia,
but can occur from overheating in a wet suit.
Over breathing to the extent that the blood carbon dioxide level is lowered,
may lead to tingling in fingers and dizziness.
A subnormal chilling of the body.
Under breathing to the extent that the blood carbon dioxide level is elevated,
may be manifested by carbon dioxide narcosis.
Lower than normal PO2 level in the blood, insufficient oxygen in the blood.
A body temperature colder than normal (98.6F), severe problems start to manifest
when body temperature reaches about 95'F.
A vertical, feet-first, method of descending into water of unknown depth or
when obstructions or heavy plant (such as kelp) growth exists; performed by
spreading arms and legs, then simultaneously bringing legs together while giving
a strong downward stroke with arms thus propelling upper body out of water;
body weight will then drive the diver downward; some also find this dive descent
easier to equalize ears because there is less blood pressure in head than with
pike (head first) dive.
Kilogram. Metric measure of weight. 1 kg = 2.21 pounds.
The velocity unit of 1 nautical mile (6080.20 ft.) per hour; equivalent to 1.689
ft. per second: to convert ft. per sec. into knots, multiply by 0.592.
Bag: After being tied to an object to be lifted, the bag is inflated
and will start to rise.
The amount of buoyancy provided by a Buoyancy Compensator; varies according
to size of the BC and according to the purpose of the BC, e.g., a BC intended
for use in cold fresh water will provide greater lift capacity than one intended
primarily for use in warm salt water.
aboard: A dive boat with sleeping and eating accommodations. Commercial
live aboard boats are usually between 50 and 130 feet long, and can carry from
10 to 30 divers for up to a week or more.
A diary of a divers dive history. Provides evidence of the depth and
breadth of a divers experience.
Volume Mask: A mask which has a smaller area between the glass and
the diver's face, usually with separate lenses for each eye; requires less air
to purge if becomes flooded.
Used on double cylinder systems. Has 2 valves similar to single tank systems
attached by a heavy duty crosspiece with a valve in the center.
lens: A lens specially designed macrophotography, enabling extremely sharp
focusing at short distances
A method of getting close-up pictures of a subject by using Marco accessories
attached to the camera's lens.
A skirted glass window constructed to provide air space between eyes and water
and to permit both eyes to see in the same plane; a regular mask covers eyes
and nose only; modern mask skirts are usually made of silicone rather than the
older rubber ones.
A painful condition when the air inside the mask is compressed by the external
pressure creating suction on the face and eyes; can be alleviated by exhaling
from the nose; can cause permanent eye damage if not equalized. Iin rapid descents
where the diver neglects to equalize his/her mask. The increase pressure causes
tissues around the eyes to swell.
Air from an over expanding lung escapes into the center of the chest. This puts
pressure on the heart and major blood vessels, interfering with circulation.
Symptoms are shortness of breath and feeling faint.
Air containing space of the ear bordered on one side by the tympanic membrane,
which is exposed to any change in ambient pressure. Air pressure in the middle
ear space can only be equalized through the Eustachian tube, which controls
the middle ear to the back of the nose.
Any non-air mixture (e.g., nitrox), although some authors use the term only
for mixes that contain a gas in addition to (or in place of) nitrogen (e.g.,
diving: A diving method whereby divers breathe special gas mixtures other than
regular air while
maximum operating depth/oxygen depth limit. The deepest that a diver can safely
go using a particular gas mixture. For example, the MOD for EAN32 (32 per- cent
oxygen) is 132 fsw (40 m).
Spending a period of time at several different depth on a single dive.
NAUI: National Association of Underwater Instructors
Also known as a "geographical mile" or "sea mile"; a unit
of distance designed to equal approximately 1 minute of arc of latitude, 6080.20
ft.; approximately 1.15 times as long as the statute mile of 5280 ft.
Inert gas that makes up 79% of air. Nitrogen is inert in that it does not enter
into any chemical reation in the body, but it can cause problems under pressure
(see nitrogen narcosis, decompression sickness).
Depressed mental state from high nitrogen pressure; usually does not begin to
manifest on compressed air until below 80 fsw.
Any mixture of nitrogen and oxygen that contains less than the 79% nitrogen
as found in ordinary air.
NOAA: National Oceanic and Atmospheric Association
An alternate second stage air source used by a diver's buddy in an out-of-air
situation, or reserve 2nd stage regulator.
OEA: Oxygen Enriched Air; Nitrox - synonym for nitrox.
Apparatus used in recreational diving. Exhaled air is expelled into the water
as bubbles, no part is re-breathed by the diver.
The recreational diving done in an environment other than a swimming pool but
with no overhead obstacles; examples include lakes, ponds, rivers, oceans, quarries.
Often seen as using the chemistry abbreviation 02, gas vital for all life on
this planet; makes up about 21% of the air by volume.
Administration of any gas, for medical purposes, that contains more than 21%
Damage or injury from inhaling too much oxygen; can arise from either too high
an oxygen concentration or oxygen pressure. One of the most dramatic manifestations
of oxygen toxicity while diving can be seizure.
Difference between total gas pressures in arterial and venous blood; exist because
oxygen is partly metabolized by the tissues, so venous oxygen pressure is lower
than arterial oxygen pressure.
A closed-circuit system which filters exhaled air, then recirculates it for
rebreathing by the diver; requires special training and maintenance.
Diving to prescribed limits, including a depth no greater than 130 fsw, using
only compressed air, and never requiring a decompression stop.
In scuba, any device that changes air pressure from one level to a lower level.
Any dive whose profile is affected by a previous dive is considered repetitive.
The time it would take to off-gas any extra nitrogen remaining after a dive.
Residual Nitrogen Time is always taken into consideration when determining the
safe duration for any repetitive dive.
Pain or discomfort in enclosed space (e.g., sinuses, middle ear, inside mask)
on ascent from a dive.
A strong current of limited area flowing outward from the shore, and may be
visible as a band of agitated water with the regular wave pattern altered; current
is caused by the rush of escaping water which is piled between shore and bar
or reef by wave action through a gap in the bar or reef; such currents are dangerous
to the uninitiated and are the cause of many drownings at ocean beaches; however,
when located by divers they are often used to facilitate entry to areas beyond
the bar or reef.
On ascent from a dive, a specified time spent at a specified depth, for purposes
of nitrogen off gassing. By definition it is not a mandatory for a safe ascent
from a dive.
The amount of salt dissolved in a liquid, measured in parts per million.
Supplied Air Snorkeling for Youth/Supplied Air Snorkeling for Adults. BC-like
PFD mounted with small compressed air cylinder and regulator that allows snorkeler
to breathe comfortably on the surface but prohibits him from descending.
submersible pressure gauge. Required scuba gear that displays the amount of
air pressure in the scuba cylinder; can be either analog or digital.
The degree to which a gas is dissolved in the blood or tissues, full saturation
occurs when the pressure of gas dissolved in the blood or tissues is the same
as the surrounding pressure of that gas.
Diving performed after the body is fully saturated with nitrogen. To become
fully saturated the diver must stay under water for a much longer period than
is allowed in recreational scuba diving tables.
surface consumption rate. Measure used in calculations for determining air consumption
rate at various depths.
Acronym meaning Self Contained Underwater Breathing Apparatus.
a place providing recreation and entertainment to vacationers with the main
attraction as that of scuba diving. It may include training, Beach and/or boat
diving and many specialties such as wall, drift, wreck, eco-diving and many
others. It can be an All-Inclusive, a Live Aboard and or a land base operation.
It will normally be supported by one or many of the training agencies.
Scuba Diving International. The recreational scuba training and certification
arm of TDI.
Flat-bottomed, plastic, vinyl or rubber devices that fit over the rounded end
of a scuba tank, allowing the tank to stand up.
Technical Diving International. Maine-based certification agency for technical
aspects of recreational scuba.
Intersection between two layers of water of that are of distinctly different
temperatures, usually the colder layer is deeper.
A part of the body characterized by specific characteristics, such as muscle,
bone, or cartilage. The term is also used to refer to any part of the body with
a specific half time for loading and unloading nitrogen or even a theoretical
Trimix: Mixture of helium, nitrogen and oxygen, used for very deep diving